1. Field of the Invention
The invention relates to a process for the preparation of o-hydroxy-benzaldehydes by catalytic oxidation of the o-cresols, on which they are based, with oxygen in the presence of chelate complexes of iron, manganese or a mixture of both.
o-Hydroxy-benzoldehydes are important intermediates for industrial syntheses of perfumes, dyestuffs, pharmaceutical, plant protection agents and photographic chemicals. The most important representative industrially is salicylaldehyde. It is used, inter alia, as a starting material for the preparation of coumarin.
2. Description of the Related Art
Salicylaldehyde is industrially accessible in several ways. The starting material is either phenol or o-cresol (Ullmanns Encyklopadie der Technischen Chemie, (Ullmann's Encyclopedia of Industrial Chemistry) 5th Edition, volume A3 (1985), p. 470). An isomer mixture of o-hydroxy- and p-hydroxy-benzaldehyde is formed in the Reimer-Tiemann reaction of phenol with chloroform in the presence of alkali (U.S. Pat. No. 3,365,500, EP-68,725). The formation of the p-hydroxy isomer is avoided by a process in which, in a first step, phenol in the form of the boric acid ester is selectively hydroxymethylated in the o-position with formaldehyde. After the cleavage of the boric acid ester, the o-hydroxy-benzyl alcohol is oxidised in a further step to salicylaldehyde with oxygen in the liquid phase on a platinum contact (DE-AS (German Published Specification) 1,261,517, DE-OS (German Published Specification) 2,612,844). Starting from o-cresol, salicylaldehyde can be prepared by side-chain chlorination and hydrolysis of the dichloromethyl group to the aldehyde group. In this process the hydroxy group has to be protected by esterification JP-73/03831, cited by C.A. 79(1973), 18387x). Further processes are based on the electrochemical reduction of salicylic acid (Ind. Chemist 39(1963), 238-41, cited by C.A. 59(1963), 10986b) and the catalytic reduction of salicyloyl halides (JP-68/13204, cited by C.A. 70 (1969), 28646j.).
The processes until now have the disadvantage that they either yield isomer mixtures or else are complicated multi-step processes in which, for example, auxiliaries such as boric acid or auxiliaries for the protection of reactive functional groups on the substances to be reacted are required.
According to EP-12,939, p-cresol derivatives can be oxidised to the corresponding p-hydroxy-benzaldehydes with oxygen in the liquid phase int eh presence of a base and of a cobalt compound in reasonable yields. This specification at the same time points out that o-cresol cannot be oxidised in this way.